1. Field of the Invention
The present invention relates to a sewing machine composed of: a main shaft; a sewing needle for forming stiches; a feed dog for feeding a work fabric to be sewn; a work feed mechanism for imparting motions in a feeding direction to the feed dog; and mechanical adjusting means for adjusting a feed pitch of the feed dog and, more particularly, to a control unit for providing seam length control to effect sewing having a length accurately coincident with a predetermined sewing length by detecting the motions of said feed dog in the feeding direction and by changing the feeding pitch of at least a final stitch.
2. Description of the Prior Art
In each of the specifications of U.S. Pat. Nos. 4,359,953 and 4,403,558, there is disclosed a control unit for controlling the seam length and the end point of work fabric by counting the number of stitches after the end of the work fabric has been detected by means of a sensor. In the specification of U.S. Pat. No. 4,404,919, on the other hand, there is disclosed in addition to the above-specified control unit a unit for changing the length of a final stitch such that the reverse mechanism of a sewing machine is operated by a micro processor on the basis of the rotational angle of an electric motor when the work end is detected. In the specification of U.S. Pat. No. 4,381,719, moreover, there is disclosed a sewing machine in which a stitch counter for setting a desired number of stitches is equipped with a correcting unit for correcting the stitch number after the detection of the work end.
Any of the seam length control units disclosed in those specifications adopts a method for controlling the number of stitches. Generally speaking, however, a work feeding mechanism is constructed by connecting a number of parts. As a result, when the sewing machine is run at a high speed, the parts themselves are warped in accordance with the increasing inertia, and tolerances and clearances between each two of the parts are deflected toward the parts having higher moving velocities until they are accumulated. This reduces responsiveness between each two of the parts to elongate the work feeding pitch. When the running speed of the sewing machine drops at the end of the sewing operation, on the contrary, the work feeding pitch is slightly shortened, as compared with the previous one. As a result, the stitch number controlling method of the prior art is accompanied by a problem that no accurate coincidence is provided between the actual seam length and a set seam length even if the practised stitch number is coincident with a set stitch number. According to the stitch number controlling method, moreover, in case the work feeding pitch is changed in the course of the sewing operation, the seam length fails to coincide with the desired value even if the sewing operation of a stitch number coincident with the set value is executed. This makes it necessary to correct again the stitch number in a manner to follow the change in the pitch, thus raising a disadvantage that the control system of the sewing machine is complicated.
In the specification of U.S. Pat. No. 4,491,080, there is disclosed a pulse generator for generating pulses corresponding to the momentum of a feed dog in the work feeding direction. This pulse generator is mounted on a shaft which is made rotatable in synchronism with the main shaft of the sewing machine. Between the former shaft and the feed dog, there is arranged a horizontal work feed mechanism for transporting the feed dog in a horizontal direction. This horizontal work feed mechanism is equipped therein with a pitch adjustor for adjusting the work feeding pitch by the feed dog. To this pitch adjustor, there is connected a potentiometer for detecting the magnitude of the pitch. This potentiometer and the aforementioned pulse generator are connected with a setting counter through a micro processor.
This micro processor calculates a desired stitch number by dividing a desired seam length by the work feeding pitch given from the potentiometer and calculates the total number of pulses to be set in the aforementioned setting counter by multiplying that stitch number by a pulse number necessary for executing one stitch. And, the micro processor subtracts the number of pulses output by the aforementioned pulse generator from that total pulse number to stop the feeding operation of the feed dog when the subtracted value becomes zero.
In the machine disclosed in that specification of U.S. Pat. No. 4,491,080, however, since the pulse generator is positioned so far apart from the feed dog that a number of parts are interposed in between, the responsiveness of any two of the parts drops to disable the pulse generator like the foregoing prior art machines to generate pulses which accurately correspond to the momentum in the feeding direction of the feed dog. As a result, the seam length controlled on the basis of the pulse number coming from that signal generator becomes slightly longer or shorter than the desired one in accordance with the running speed of the sewing machine. In this machine, moreover, since the pitch adjustor is interposed between the feed dog and the pulse generator, in order to set the aforementioned setting counter with an accurate total pulse number corresponding to the desired seam length, it is necessary to monitor the magnitude of the pitch at all times by the potentiometer and to input again that value to the micro processor, and this necessity disadvantageously complicates the construction and program of the control system.